1. FIELD OF THE INVENTION
The present invention relates to an apparatus for generating electrical isometric control signals resolving applied forces in six degrees of freedom. In particular, it relates to a joystick control apparatus for three translational and three rotational degrees of freedom.
2. PRIOR ART OF THE INVENTION
Isometric control devices of the force operated or "stiff-stick" type have been known and in use for some time. These are useful in manually controlling motion in three dimensions in applications such as tracking systems, machine control and vehicular control. They are also useful for inputting information into computers in connection with three-dimensional computer graphics. The device is also known as a "joystick".
A joystick typically comprises a control rod or shaft fixed at one end, and on which are mounted strain gauges to measure the movement of the rod upon the application of forces. An electronic circuit processes the strain gauge signals and generates output signals representative of the force applied to the free end of the rod. In the past, such techniques and apparatuses were limited to four degrees of freedom. The strain gauges were typically arranged in pairs and were either parallel to the longitudinal axis of the control rod or at an angle. The known configurations did not permit three dimensional control in six degrees of freedom.
U.S. Pat. No. 3,454,920, issued July 8, 1969 Morton H. Mehr for an invention entitled "Isometric Control Device", describes a device for sensitive control purposes utilizing strain gauges and a shaft with a fixed end, the free end being operable by the thumb or finger as compared to its prior art where "stiff-stick" type controllers were. usually large and required handgrips. The patent describes use of a plurality of annularly arranged strain gauges on the shaft to give accurate control. A further aspect of the invention was the use of strain gauges comprising solid-state semi-conductors.
A three axis joystick control is described in U.S. Pat. No. 4,046,005, issued Sept. 6, 1977 to Robert D. Goroski. The patent discloses a fixed end control rod with strain gauges mounted on the rod in such a configuration as to provide signals for control in three degrees of freedom. The strain gauges are mounted in opposed pairs on the control shaft at angles to the shaft axis in order to provide control in three axis. The prior art devices resolved rotational forces or torque only about the shaft axis.
The resistances of certain types of strain gauges must be measured to measure applied forces. The change in resistance may be small, since if the control apparatus is to be robust and reliable it should be operated at low strains. If individual circuitry is used to sense and amplify each strain gauge signal, severe restraints must be placed on the stability of gain and zeroing to obtain a reasonably accurate output.
Semi-conductor strain gauges have been used to mitigate this problem; such gauges give a high output at the expense of increased fabrication costs and temperature sensitivity in comparison to a foil gauge. So extreme is the temperature sensitivity of semi-conductor gauges that small temperature differences between the ends of the control rod, as might be caused by the heat of the operator's hand, may cause substantial errors. Foil or wire gauges, less sensitive to strain, are much less sensitive to temperature, and give a better ratio of strain sensitivity to temperature sensitivity.